Buffer circuits (e.g., output buffers and bidirectional buffers) are employed in a variety of electronic devices and applications. There are traditionally digital buffer circuits and analog buffer circuits, each having their own advantages and disadvantages associated therewith. One advantage of digital buffer circuits is their ability to source or sink relatively large currents (e.g., about several tens of milliamperes (mA) or higher), thereby providing a substantially low output impedance drive. This is due, at least in part, to the fact that full-scale digital signals (e.g., VSS to VDD) are used to drive output stages in the digital buffer circuits.
Under certain conditions, however, the low output impedance provided by a given digital buffer circuit presents a problem. For example, when a contention condition arises, which may occur when a logic “high” output of the buffer circuit is somehow shorted to VSS, a large “over current” will be sourced by the output stage in the buffer circuit. Likewise, when a logic “low” output of the buffer circuit is somehow shorted to VDD, a large over current will flow into the output stage of the buffer circuit. This large over current, which may be about 50 mA or higher, can cause reliability problems due, at least in part, to electromigration damage in the metal connecting the output stage devices to a supply voltage and to an output pad. The large over current can also damage the output stage devices themselves.
One conventional solution to reduce electromigration damage resulting from the large over current during a contention condition is to substantially increase the size of the metal connections in the output paths of the output stage devices in the buffer circuit. However, increasing the size of the metal connections significantly increases the overall size of the buffer circuit, and is thus undesirable. Additionally, while it is desirable to be able to limit the current sourcing (or sinking) capability at the output of the buffer circuit under such a contention condition, there is no known means for providing current limiting while at the same time assuring a low output impedance drive under normal operation of the buffer circuit (e.g., when no contention condition exists).
An advantage of analog buffer circuits is that, unlike digital buffer circuits, they generally provide current limiting. This is due primarily to the fact that the output current sourced or sunk by an output stage of an analog buffer circuit is typically generated from a current mirror arrangement, which is biased to some fixed reference current source. Therefore, analog buffer circuits are generally not prone to reliability problems and/or damage resulting from a contention condition. However, because the current sourcing and sinking capabilities of the analog buffer circuit are typically limited, an output impedance drive of the analog buffer circuit is often substantially higher than that of a comparable digital buffer circuit. Depending on the application in which the buffer circuit is employed, the output impedance drive provided by an analog buffer circuit may not be low enough to meet certain output impedance specifications.
Accordingly, there exists a need for an improved buffer circuit that provides a substantially low output impedance and yet does not suffer from one or more of the problems exhibited by conventional buffer circuits.